Evaluation And Mitigation Of Rain Vibration Of Stay Cables

Randall W. Poston, Ph.D., P.E., WDP & Associates, Inc.

Cable-stay bridges have become increasingly popular for bridges requiring longer spans. In recent years, large amplitude vibrations of stays have been observed on cable-stay bridges in Japan, Europe and North America. This is a relatively new phenomenon since the vibrations occur under moderate and light winds with rain. This type of vibration referred to as “rain vibration” is of concern to bridge engineers and owners because the vibration can lead to fatigue damage of structural elements and to serviceability problems. Additionally, the vibrations are worrisome to passing motorists.

Since 1997, this type of rain vibration has been observed at the two cable-stay bridges in Texas. An evaluation and monitoring program was implemented to evaluate the conditions under which the vibrations occur and to assess non-destructively if significant damage of the stay cables had occurred. This damage assessment was conducted using vibration monitoring techniques to determine effective stay tensions.

Based on the evaluation and monitoring program, several vibration mitigation strategies were selected for prototype study. These have included two types of viscous dampers and cross-ties or restrainer cables. Extensive monitoring was conducted to assess the effectiveness of the vibration mitigation strategies.

As part of the investigation, various methods were studied to mitigate stay vibrations. In one form or another, all involved increasing damping in the stays.

Cable Restrainers
Cross-ties or cable restrainers have been used primarily as a temporary solution to cable stay vibration problems. A review of the literature provided little or no details of their placement, number or design. Thus the approach utilized for the Hartman Bridge was phenomenological.

A cable restraint system configuration using three lines of restrainer cables was developed. Analyses were performed on various three-line restrainer configurations. These analyses generally met the design requirement of increasing the stay cable and stay cable substructure frequencies to a value greater than the target value of 2 Hz. The configuration implemented is shown schematically in Figure 22 along with the principal mode shape of the stay-restrainer system.

Figures 23 and 24 illustrate the significant decrease in measured stay accelerations of a stay on the Fred Hartman Bridge before and after the cross-ties were installed. Clearly, this has been an effective vibration mitigation solution. Although this method has been proven to be effective, this method is temporary and a permanent active system was ultimately developed.

Hydraulic Dampers
Retrofit hydraulic dampers were the principal vibration control measure investigated for the permanent mitigation of rain vibration of the stay cables. The dampers act similar to automotive shock absorbers, absorbing the vibratory motion of the stay cables (motion is absorbed as strain energy), and providing additional damping to prevent further motion of the stay cable. To evaluate the potential use of retrofit dampers, prototype hydraulic dampers were developed for both the Fred Hartman and Veterans Memorial Bridges.

The dampers that were designed for the two bridges were based on a procedure developed by Pacheco et al. (1993). The procedure was developed from analysis of a taut cable with a single damper near the anchorage. In the procedure, the damping constant of the retrofit damper is developed from a design curve which take into account the cables natural frequency, mass, length and damper location. The damper mechanism is then designed using the damping constant, natural frequency and expected motions of the damper to determine the required stroke, power, and velocity characteristics of the damper. The prototype dampers were attached into the bridge deck by means of an inclined, double-angle strut. Figure 25 shows a prototype damper that was evaluated for the Fred Hartman Bridge.

Figure 26 shows the significant decrease in measured accelerations of a stay with the hydraulic damper installed (compare to Figures 23). Because of the positive results from the results of the prototype damper over more than a two year period, the permanent vibration mitigation solution will be dampers to be installed sometime this year (2002).

Acknowledgements
The contents of this article reflect the views of the author who is responsible for the facts and accuracy of the data presented. The contents do not necessarily reflect the views or policies of TxDOT. The author acknowledges the contributions of Dr. Nick Jones of Johns Hopkins University, Baltimore Maryland, for the vibration data collected as part of the overall research program funded by TxDOT.

References
Pacheco B., et al., (1993), “Estimation Curves for Modal Damping in Stay Cables with Viscous amper”, Journal of Structural Engineering, ASCE, pp. 1961-1979, June.

Editor’s Note: Randall W. Poston will present a paper on this investigation of means of controlling stay cable vibrations at the November 18-19 ASBI Convention in San Francisco.